A solar cell module generally has a structure in which from the light-receiving surface side to which sunlight is incident, a glass or front sheet/a transparent filling material (sealing material)/a solar cell element/a sealing material/a back sheet are laminated in this order. Specifically, the solar cell element is generally embedded with a resin (sealing material) such as EVA (an ethylene-vinyl acetate copolymer) and the like, and a protective sheet for a solar cell is adhered thereto. The protective sheet for a solar cell, above all, a back sheet for a solar cell module, which becomes in particular an outermost layer, is considered to be under an environment exposed to weather outdoor or direct sunlight for a long period of time, and therefore, excellent weather resistance (wet heat resistance, heat resistance) is required.
As the back sheet for a solar cell module, in the related art, a polyester film, in particular, a polyethylene terephthalate (hereinafter, referred to as PET) film has been used. The polyester film has excellent heat resistance, mechanical characteristics, chemical resistance, and the like, and therefore, the polyester film is widely used industrially. However, since the film has poor hydrolysis resistance, the molecular weight is reduced by hydrolysis, and mechanical characteristics are reduced by progressively stiffening. Therefore, it was not possible to maintain a practical strength for a long period of time as a back sheet for a solar cell.
In order to solve the above problems, for example, Patent Document 1 discloses the use of a ketene imine compound as a terminal blocking agent of the polyester in order to suppress hydrolysis of polyester. Here, it is proposed that hydrolysis of polyester is suppressed by reacting the ketene imine compound with the terminal carboxyl group of polyester.